Re: SHA1 hash safety

[David Lang <david.lang@digitalinsight.com>]

On Sat, 16 Apr 2005, Ingo Molnar wrote:

> * David Lang <david.lang@digitalinsight.com> wrote:
>
>> this issue was raised a few days ago in the context of someone
>> tampering with the files and it was decided that the extra checks were
>> good enough to prevent this (at least for now), but what about
>> accidental collisions?
>>
>> if I am understanding things right the objects get saved in the
>> filesystem in filenames that are the SHA1 hash. of two legitimate
>> files have the same hash I don't see any way for both of them to
>> exist.
>>
>> yes the risk of any two files having the same has is low, but in the
>> earlier thread someone chimed in and said that they had two files on
>> their system that had the same hash..
>
> you can add -DCOLLISION_CHECK to Makefile:CFLAGS to turn on collision
> checking (disabled currently). If there indeed exist two files that have
> different content but the same hash, could someone send those two files?

remember that the flap over SHA1 being 'broken' a couple weeks ago was not 
from researchers finding multiple files with the same hash, but finding 
that it was more likly then expected that files would have the same hash.

there was qa discussion on LKML within the last year about useing MD5 
hashes for identifying unique filesystem blocks (with the idea of being 
able to merge identical blocks) and in that discussion it was pointed out 
that collisions are a known real-life issue.

so if collision detection is turned on in git, does that make it error out 
if it runs into a second file with the same hash, or does it do something 
else?

David Lang

-- 
There are two ways of constructing a software design. One way is to make it so simple that there are obviously no deficiencies. And the other way is to make it so complicated that there are no obvious deficiencies.
  -- C.A.R. Hoare